Leak sensor

ABSTRACT

A dressing is provided for application to an object that is, at least partly, electrically conductive. The dressing includes an adhesive for attaching the dressing to the object, and at least two electrodes. The electrodes are adapted to be arranged at a distance from the partly electrically conductive object so that a first capacitor is formed between a first electrode and the partly electrically conductive object, and a second capacitor is formed between a second electrode and the partly electrically conductive object.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method, a dressing and a dressingassembly for detecting a leak between the dressing and an electricallyconductive object by measuring the changes in capacitance between thedressing and the electrically conductive object.

2. Description of the Related Art

Some stoma patients experience problems with leakage from ostomy bags orleakage of body fluids from the ostomy. Origins of the leakages can beskin folds developed underneath the adhesive of a bandage, a scar near astoma not providing good adhesion or presence of a humid spot prior toattaching the adhesive and thereby preventing the adhesive fromobtaining good adhesion to the skin or the leak may occur in time as theadhesive properties of the bandage deteriorate. A leakage for a stomapatient can be embarrassing and highly inconvenient. Also wound carepatients can have problems with leakages from a dressing. A patient maywithdraw from social contact because of the smell or the sight ofexudates leaking from a wound.

With an increasing elderly population in many countries, national healthcare systems have much focus on the total cost. For stoma patients orchronic wound patients, the bandages or the dressings are often changedregularly by a caring nurse. Extending the length of time a bandage or adressing can remain on the stoma or wound reduces the need for care,enabling a reduction in health care costs.

An early warning reporting system either to the patient or to the caringnurse that a bandage or a dressing needs to be changed would thereforebe useful. Not only will the patient avoid unpleasant accidents, helpingthe patient to feel confident and secure in his or her daily life, butit may also reduce the cost of our health care systems.

There is therefore a need of a sensor, which is able to report to thepatient himself or herself or to a caring nurse, when leakage starts tooccur in a bandage or a dressing and it needs to be changed. However, itis important that the sensor triggering such an alarm has a high degreeof reliability, i.e. essentially no false negative alarms, and that itcan provide a fast response enabling the patient or the nurse sufficienttime to change the bandage or the dressing.

U.S. Pat. No. 4,754,264 discloses a water detecting device for a diaperbased on sensing the degree of wetness by positioning two capacitorelectrodes in the diaper detecting a liquid being absorbed by a waterpermeable sheet covering the two electrodes. The water permeable sheetcauses a change in the dielectric constant of the electrostaticcapacitance between the two electrodes and a detecting circuit detectinga change of the electrostatic capacitance transmitting a signalindicating that the diaper is wet. The U.S. Pat. No. 4,754,264 makes useof the fact that a medium comprising a water absorbing material getswet, but the water has flown from the source to the permeable sheet,before it gets absorbed. It is therefore not suitable to detect a leakbefore it has wetted the medium.

WO02/099765 discloses a resonance circuit arranged on an insulatingsubstrate with at least one pair of spaced measuring electrodes arrangedin parallel or in series with the resonance circuit in such a mannerthat a material able to change its electrical conductivity underexternal influence can be placed or can pass through in the spacebetween the measuring electrodes. Measuring the change of the resonancefrequency or the Q-factor of the resonance circuit permits measuring andsurveying the electrical conductivity of a material and measure changesunder external influence such as e.g. moisture and temperature. However,the invention makes use of a change in the electrical conductivity andthe material to be measured needs to be placed between the two measuringelectrodes. This method is therefore not suitable for detecting a leakat the interface between the skin and an adhesive, because thisinterface cannot in a practical way be placed between two measuringelectrodes.

EP 1 188 157 discloses a radio frequency resonant circuit sensing devicefor detecting a fluid in an article or container, in particular a bodyfluid from a human or an animal. The device comprises a layeredstructure with two electrically conducting electrodes attached to asubstrate and with a dielectric medium in between the two electrodes.The resonant circuit sensing device is contained within the article tobe measured. The invention can be suitable for measuring if a woundbleeds or a stoma bag is filled. However, the body fluid to be measuredneeds to be guided to the space between the two electrically conductingelectrodes. It is therefore not suitable for detecting a leak due toexudates of a wound or discharge from a stoma propagating between theskin and an adhesive to the periphery of a bandage or dressing.

GB 2 343 628 discloses a device measuring the conductivity of theadhesive of a seal for an ostomy bag. The measuring device comprises twoseries of microelectrodes arranged on two tracks of at least an arc of acircle whose centre is coincident with the centre of a through-passageto an ostomy. The microelectrodes have substantially the form of a smallcylinder of metal and penetrate to about half the thickness of theadhesive composition. It is thus the conductivity of the adhesivebetween the two series of electrodes, which is measured.

However, such a device measuring the conductivity of the adhesive onlymakes a respond in the case, where the adhesive has lost itseffectiveness and it is desirable to change the adhesive. Furthermore,as the device measures directly between the two series of electrodes,the device will not detect a leak occurring as a consequence of askin-fold, a scar or a wet spot creating a channel for a leak and wherea leak propagates between the skin and the adhesive.

There is therefore a need in the art of sensors of being able to detectleaks around the whole periphery of a stoma or a wound propagatingbetween the skin of a person and the adhesive of a bandage or a dressingin sufficient time before such a leak has propagated to the edge of thebandage or dressing.

An objective of the present invention is to provide a sensor detecting aleak between an adhesive of a bandage or dressing and the skin of aperson or any other mammal making use of the skin as a ground plane foran alternating electrical current at frequencies, where the skin has amoderate or high conductivity.

Another objective of the present invention is to detect the leak inample time before it has propagated completely to the edge of thebandage or dressing.

A further objective of the present invention is to provide sensitivityat the whole periphery of a stoma or a wound.

An even further objective of the present invention is to provide asensor, which may be produced at high production throughput and at lowcost.

An even further objective of the present invention is to provide asensor which is disposable and which may be connected to a reader unitin a simple way.

An even further objective of the present invention is to provide asensor reporting the condition of the adhesive of a bandage or adressing of a wound to a patient, a caring nurse or a service centre.

SUMMARY OF THE INVENTION

Thus the present invention discloses a method for detecting detachmentof a dressing, for example a bandage, patch or ostomy base plate, whichis applied to a surface of an at least partly electrically conductiveobject, said dressing comprising an adhesive for attaching the dressingto the electrically conductive object and at least two electrodesarranged at a distance from the electrically conductive object, andwherein, a voltage is applied to the first and second electrodeestablishing an electrical circuit comprising a first capacitor betweenthe first electrode and the electrically conductive object and a secondcapacitor between the second electrode and the electrically conductiveobject; changes of the capacitance between at least one of the first andthe second electrode and the electrically conductive object is detected;and an alarm is activated when the changes of the capacitance reach apredetermined value.

This advantageously provides a method whereby a leak can be detectedquickly.

Furthermore, this advantageously provides dressing having a leak sensorwith no direct physical or mechanical contact between the electricallyconductive object, e.g. the skin, and the electrodes. This provides fora dressing comprising a leak sensor without compromising the comfort ofthe user.

When read in this text, the term ‘conductive’ should be read as‘electrically conductive’ unless explicitly stated otherwise.

It should be understood that ‘an at least partly electrically conductiveobject’ may be an object comprising areas which are not conductive,while other areas are conductive, e.g. the skin of a person having asurface not being conductive except below that surface. Furthermore ‘anat least partly electrically conductive object’ may also be an object,which is only conductive when certain electrical signals are applied.Again, the skin of a person is an example where applying a directcurrent (DC), the skin practically exhibits no electrical conductivity.However, applying an alternating current (AC), the skin exhibitselectrical conductivity, which is increasing several decades withincreasing frequencies in the range from DC to approximately 100 MHz.

The electrodes can be formed of many different types of materials, forexample metals like silver, gold, aluminium or copper or paste of silveror aluminium; conducting polymers like polyaniline, polypyrrole,ethylenedioxythiophene, poly(p-pyridyl vinylene); or amorphousconducting carbon films, films of conducting carbon fibres orpolymer-conducting-carbon-black. Materials may also be alloys and/orsemiconductors such as tin oxide (SnO₂), zinc oxide (ZnO₂), indium tinoxide (ITO) or the like.

Furthermore, the term ‘alarm’ should be understood broadly as being anytype of means for drawing attention to the dressing. Such an alarm maybe present on the dressing itself, such as a small vibrator, sound orlight emitting element or a color indication. Alarms remote from thedressing may also be provided, for example personal computers, cellularphones or PDAs (Personal Digital Assistants) or iPods, which all arecapable of using known means in order to obtain the attention of aperson.

The predetermined value of the change of capacitance on which the alarmis activated may be of different types. For example, an alarmcorresponding to a leak may be an increase in the magnitude of thecapacitance, or a decrease in the magnitude of the capacitance, or itmay be a time variation of the change of capacitance. Such a timevariation of the change of capacitance may be an initial negative changeof capacitance within a first predetermined period of time following bya positive change of capacitance within a second predetermined period oftime. An alarm corresponding to a soaked adhesive may be a positivechange of capacitance within a third predetermined period of time.

Typically, said first and said second predetermined period of time areshorter than said third predetermined period of time. Said first andsaid second predetermined period of time may be within 10 seconds to 30minutes and said third predetermined period of time may be within 10minutes to 100 hours or longer. Activation of alarms may be triggered byother types of predetermined values of the change of capacitance. Theterm predetermined value is thus not limited to a single value, but mayalso include a two-dimensional array of time and values of changes ofcapacitance. It may also include arrays of other variables such aschanges of temperature, pressure and the like. The other variables maybe measured by integrating other sensors such as temperature sensors andpressure sensors in the dressing and/or in the reader measuring changesof capacitance.

The terms ‘dressing’ and ‘bandage’ have been used alternating within thepresent text. Within the scope of the present invention these termsshould be read identically. In general they both refer to an assembly ofan adhesive, which is disposed on a backing layer. Said assembly canfurther comprise numerous additional components. For example theadhesive will typically be skin friendly as the assembly is to beapplied to the human body. In some cases absorbent materials are addedeither in the adhesive or inserted between adhesive sections of theassembly, e.g. for wound dressings in order to absorb fluid from awound. Hydrocolloid particles can also additionally/alternatively beadded in the adhesive compound in order to further improve the adhesiveand skin friendly properties of the adhesive. Such hydrocolloidadhesives are frequently used in a body side wafers within ostomyappliances, whether it is so-called one-piece or two-piece appliances. Abody side wafer is applied to the skin around a stoma and holds thecollecting bag for collecting stools from the stoma.

Furthermore, it should be understood, as would be appreciated by theperson skilled in the art, that the terms resistance, capacitance andinductance all indicates a part or component having a numerical value,thus, throughout the application these may be realised by providing aresistor, capacitor and inductor, respectively.

In one embodiment, the changes of the capacitance may be detected bymonitoring the frequency response of the electrical circuit. Allresonance circuits, i.e. electrical circuits comprising a capacitanceand an inductance, have a frequency response and such resonance circuitscan be obtained by using few very simple components. Thus, a simplecircuit comprising a few and low cost components may be provided inorder to detect a leak, by transducing the leak into a time response orfrequency response, which readily can be monitored.

The frequency response may be monitored using different methods. In oneembodiment the frequency response of the electrical circuit is carriedout by performing a frequency sweep at regular intervals while measuringthe response from the electrical circuit. In another embodiment, themonitoring of the frequency response of the electrical circuit can bemade applying an electrical impulse to the electrical circuit andsubsequently measuring the response from the electrical circuit.

In another aspect of the invention, a dressing suitable for applying themethod and its embodiments described herein is furthermore disclosed.Such a dressing according to the invention comprises an adhesive bodyhaving a proximal adhesive surface; at least a first and a secondelectrode arranged on the dressing so that at least a part of theadhesive body is arranged between the proximal adhesive surface and thefirst and second electrodes, wherein a first width of the firstelectrode and a second width of the second electrode are larger than therespective first thickness and second thickness of the respective firstand second electrode in at least one area.

This allows for flat, very thin and compact dressings whereon theelectrodes may be provided. This is possible since the capacitance isinfluenced by the width of the capacitor, i.e. the area of the surfaceparallel to the surface of the conductive object, and not the thicknessof the electrodes. Thus, very thin electrodes may be provided. Thisadvantageously allows for discreet and comfortable dressings, suitablefor daily wear beneath the wearer's clothes.

It should be understood that the adhesive body exhibits primarilydielectric properties, i.e. shows a very low electric conductivity, inorder to establish a capacitance between an electrode and theelectrically conductive object, e.g. the skin of a person. Typically,before use the adhesive body has a “dielectric” value between 2 and 3.

For the adhesives of the embodiments of the present invention, theresistance of the adhesive between electrodes to the adhesive surface ofthe dressing, which is attached to the at least partly electricallyconductive object like the skin of a mammal, is approximately given by

${R_{adh} = {\rho_{adh}\frac{_{adh}}{A}}},$

where ρ_(adh) is the resistivity the adhesive, l_(adh) is the thicknessof the adhesive, and A is the area of the first electrode. Anon-conductive adhesive means that the adhesive exhibits dielectricproperties and that R_(adh) is larger than the other contributions tothe resistance measured from any position on the first electrode to anyposition on a second electrode on top of the adhesive. Othercontributions to the resistance are the resistance of the skin([R_(skin1)+R_(skin2) in FIG. 2( b)]) and the resistance of theelectrode. Typically, this means that the resistivity of the adhesive islarger than 0.1 Ωm, more preferably larger than 10² Ωm, and even morepreferable larger than 10⁶ Ωm. For embodiments, where a foil is disposedin between the adhesive and the first and second electrodes, the foilshould be considered as a part of the adhesive with the samerequirements for being non-conductive.

In one embodiment, the first and second width is uniform along theextent of the first and second electrode, respectively. This providesfor uniform measurements independent of the position of the occurringleak.

In order to be able to detect a leak occurring around an area ofinterest, the second electrode at least partly encircles the firstelectrode. Additionally or alternatively at least one of the electrodesshould enclose the area of interest in order to detect a leak before itenters, or alternatively exits, said area of interest.

The electrodes may be formed in many different forms and shapes. Forexample, they may be formed as tracks having many different paths, forexample any known geometrical shape such as oval, square, triangular oralternatively have an irregular shape. In one preferred embodiment thefirst and the second electrode are formed as respective first and secondcircular track and where the inner diameter of the second circular trackis larger than the outer diameter of the first circular track. Thisallows the electrodes to be simple to produce and that a signalrepresenting a leak be the same independently of where the leak occursalong the electrodes.

In another embodiment of the dressing, an inductance may connect thefirst and second electrode. This provides an inductance whereby aresonance circuit is established and it may be possible to induce acurrent from a distance through the inductance, typically in the shapeof a coil, thereby enabling wireless electromagnetic coupling to areader circuit.

In an alternate embodiment, the first and the second electrode areelectrically isolated from the adhesive body. Thus, direct contactbetween the electrodes and body liquids absorbed by the adhesive can beavoided.

In order to achieve a capacitance between the conductive object and therespective electrodes, the electrodes have to be arranged in a distancefrom the conductive object. Thus, in one embodiment the at least firstand second electrode are arranged at least partly embedded in theadhesive body and in another embodiment the at least first and secondelectrode are arranged on the distal side of the adhesive body.

When the electrodes are at least partly embedded in the adhesive, aconstruction may be provided wherein the conductivity of the adhesivefurthermore can be measured and such constructions are additionally oralternatively also solid and may be discretely hidden in the adhesive.

When the electrodes are arranged on the distal side of the adhesive,i.e. the opposite side of the proximal adhesive surface which isattached to the conductive object, the electrodes may be made easyaccessible for different purposes such as measurement, replacement,monitoring etc.

In one embodiment, the first electrode and the second electrode areprinted on a flexible film. This is one simple way of providing adressing according to the invention as the film subsequently can beattached to the distal side of the adhesive body.

In another embodiment, an encircling groove is formed in the proximaladhesive surface of the adhesive body, encircling the centre of thebandage or dressing. The encircling groove may be arranged opposite oneof the electrodes. This enhances the sensitivity of detecting a leak, asthe response from the electrical circuit changes significantly when thecontent of groove changes, such as being filled with a liquid.

Furthermore a first electrically conductive ring and a secondelectrically conductive ring may be arranged in the encircling groove,and when the first electrically conductive ring is electricallyconnected to the first electrode and the second electrically conductivering is electrically connected to the second electrode. A liquid fillingthe groove will practically create a short circuit or a strongcapacitive coupling between the first conductive ring and the secondconductive ring, which significantly will change the response from thecircuit and thereby identifying a leak.

It should be understood that the invention disclosed herein is notlimited to only two electrodes. Thus in another embodiment a third ormore electrodes may encircle the first and the second electrode. Amultitude of electrodes on the distal side of an adhesive may beadvantageous for the detection of a leak. Each time the leak is below aparticular electrode, a capacitance change related to that electrode maybe detected. The scheme therefore enables detection of the course of apropagating leak.

In one embodiment according to the present invention, the dressing is abase plate of an ostomy bag. Thus means are provided to detect fluidleaking between the base plate and the skin, and it may be detectedpractically as soon as a leak occurs or even prior to the occurrence.

In order to eliminate ambient noise, a first upper electrode and asecond upper electrode electrically connected by a second inductance canadvantageously be arranged on the distal side of the first and secondelectrode. The first upper and the second upper electrode may recordambient disturbances such as capacitive coupling via a fluid in theostomy bag and the signal from such disturbances may be subtracted fromthe signal coming from the first and second electrode thereby creating aresulting signal discriminating a leak from other events.

An additional way of eliminating ambient disturbances is introducing anelectrically conductive shield layer on the distal side of the first andthe second electrode with a dielectric medium between the shield layerand the electrodes.

In yet a different embodiment, the change in capacitance may be detectedby monitoring the change in the time constant of the establishedelectrical circuit. The time constant may for example be detected by acounter and an astable multivibrator, which is connected to the firstand second capacitor via at least one resistor, thereby forming aresistor-capacitor circuit. It is well known that a time constant may bedetermined from such a circuit.

Other means for detecting the time constant may be provided. Forexample, instead of an astable multivibrator, the time constant of theelectrical circuit may be detected by a Colpitts oscillator comprisingthe first and second capacitor and at least one inductor, andrepetitively counting the change in the number of output pulses from theColpitts oscillator within a predetermined time interval.

Thus, it may be understood by the person skilled in the art that manydifferent types of oscillators may be used to arrive at the claimedinvention.

In another aspect of the invention a shield for reducing capacitivecoupling from ambient surroundings into the electrodes of the dressingis provided as described above. Such a shield is advantageously formedas an electrically conductive pattern, which is provided on the distalside of the at least first and second electrodes. Thus, more reliabledetection of coming or occurring leaks can be provided.

It should be understood that by the distal side of the first and secondelectrodes it is referred to the side facing away from the surfacewhereon the dressing is attached.

Advantageously, when the dressing is a base plate connectable to anostomy collection bag, the electrically conductive pattern can bearranged between the at least first and second electrodes and the ostomycollection bag to shield from capacitive coupling between the electrodesand the contents in the collection bag.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is disclosed more in details with reference theembodiments in the drawing in which,

FIG. 1 a shows a perspective view of an embodiment of a dressingaccording to the present invention,

FIG. 1 b shows partly a cross-sectional view of the embodiment from FIG.1 a and a leak,

FIGS. 2 a, 2 b and 2 c show the embodiment of FIGS. 1 a and 1 b, theelectrical equivalent diagram thereof and schematically a plot of thefrequency response change as a result of the change of capacitance,

FIGS. 3 a, 3 b, 3 c, 3 d, 3 e, 3 f, 3 g and 3 h show four alternativeembodiments of dressings according to the invention both in top viewsand in cross-sectional views,

FIGS. 4 a, 4 b, 4 c and 4 d show even further alternative embodiments ofdressings according to the present invention,

FIG. 5 shows an alternative embodiment of a foldable electrical circuitsuitable to be disposed on a standard ostomy base plate,

FIGS. 6 a and 6 b show an alternative embodiment of a dressing accordingto the invention in an exploded and assembled perspective view,respectively,

FIGS. 7 a and 7 b show two different embodiments of reader unitssuitable for detecting changes in capacitance according to the presentinvention,

FIGS. 8 a and 8 b shows two different embodiments of a communicationsystem for providing remote monitoring of a dressing according to theinvention,

FIG. 9 shows a plot of an electrical response as a function of frequencyfor an embodiment of the present invention attached to the skin of aperson,

FIG. 10 shows a plot of the resonance frequency as function of time foran embodiment of the present invention attached to the skin of a person,

FIG. 11 shows a plot of the resonance frequency as a function of timefor an embodiment of the present invention attached to the skin of aperson,

FIG. 12 shows a plot of the response before the occurrence of the leakand a plot of the curve after the occurrence of the leak for anotherembodiment of the present invention attached to the skin of a person,

FIG. 13 a, 13 b and 13 c show an electrical block diagram and drawingsof another embodiment of an electrical circuit applicable for thepresent invention,

FIGS. 14 a and 14 b show a plot indicating a leak as illustrated by theresonance frequency as function of time for two different embodiments,respectively, of a dressing according to the invention attached to theskin of a person,

FIG. 15 a, shows an embodiment of an ostomy appliance according to theinvention,

FIG. 15 b, 15 c and 15 d show different embodiments of an ostomyappliance according to the invention illustrating means of reducing theinfluence of capacitive coupling from outside sources,

FIGS. 16 a and 16 b show effects of capacitive coupling between theenvironment above the top-foil and the electrical conductors as plots ofthe resonance frequency as function of time for an embodiment of thepresent invention where no shield layer is provided on a dressingaccording to the invention and where a shield layer has been provided ona dressing according to the invention, respectively, and

FIG. 17 a, 17 b, 17 c and 17 d show an alternative embodiment of abandage of the present invention with a top view of the conductivepatterns for the two ring capacitors, the electrical shield layer, thealignment of the conductive patterns for the two ring electrodes, and incross section the embodiment along the line A-A.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

FIG. 1 a shows schematically a bandage 1 attached to the skin of amammal 2 comprising an opening 3, an adhesive 4, a top-foil 5 and apattern of electrically conductive films 6. FIG. 1 b is a partialcross-sectional view of the bandage with a leak 10 propagating from astoma or a wound along the interface between the adhesive 4 and thesurface of the skin. A gap 11 may also be created prior to thepropagating leak. The skin is illustrated by an outer layer epidermis 24of low conductivity and a middle layer of dermis with higherconductivity 25. The inner layer of hypodermis below the layer of dermisis not illustrated in FIG. 1 b.

The bandage 1 may be used as an adhesive base plate for an ostomy bag(not shown). The base plate thus has an opening 3 to allow dischargefrom a stoma (not shown) to fill the ostomy bag attached to the baseplate. In another embodiment, the bandage may be used as a dressing fora wound (not shown), wherein the opening 3 surrounds the wound and thedressing containing antibacterial and wound-healing compounds are placedin the opening 3 with the purpose of covering the wound.

It should however be understood that the present invention is notlimited to base plates in ostomy and dressings in wound care, but coversother forms of detection of leakage of body fluids. An example is afecal collector, which includes a bag having an opening for receivingfecal discharge and an attachment patch to be attached to the skin of apatient. Such a fecal collector is described e.g. in U.S. Pat. No.5,593,397. The present invention could readily be integrated in theattachment patch of the fecal collector to provide a warning report of afecal leakage.

When an alternating current is applied to the electrically conductivepattern, capacitive coupling occurs between the electrically conductivepattern on the top-foil and the skin. The conductive pattern 6 has aninner ring-capacitor 7, a coil 8 and an outer ring-capacitor 9. Thetop-foil 5 separates the electrically conductive film 6 from theadhesive 4. If the adhesive absorbs liquid, the top-foil prevents theliquid from being in physical contact with and short-circuiting theelectrically conductive films.

FIG. 2 a shows a top-view of the bandage from FIG. 1 a. FIG. 2 b shows afirst order electrical equivalent diagram with discrete componentsdepicted. It is the equivalent diagram of the electrical circuit whenthe bandage is applied to the skin of a mammal, for example as in FIG. 1a. FIG. 2 c shows schematically a plot of the power dissipated in thecircuit diagram in FIG. 2 b as a function of the applied frequency froman electromagnetic field coupling to the coil of the electrical circuit.In FIG. 2 c, curves are plotted in the case of no leak 18 and in thecase of a leak 18′. The discrete components represent the capacitanceC_(w) 12 between the wires of the coil 8 and between the two ringcapacitors 7 and 9, an inductance L_(c) 13, a series resistance R_(c)14, capacitance C_(adh1) 15 between the inner ring of the electricallyconductive film and the skin, capacitance C_(adh2) 16 between the outerring electrically conductive film and the skin, and the resistance ofthe skin R_(skin1) 17 and R_(skin2) 17′. Neglecting the resistance ofthe electrically conductive films R_(c), and the capacitance between thewires and the two capacitive rings C_(w), the resonance frequency isapproximately given by

$\begin{matrix}{{f_{res} = \frac{1}{2\pi \sqrt{L_{c}\; C_{tot}}}},} & {{Eqn}.\mspace{14mu} 1}\end{matrix}$

where C_(tot) is the series connection of the two capacitors C_(adh1)and C_(adh2)

$C_{tot} = {\frac{C_{{adh}\; 1}C_{{adh}\; 2}}{C_{{adh}\; 1} + C_{{adh}\; 2}}.}$

An approximate expression of the capacitance C_(adh1) reads

$\begin{matrix}{{C_{{adh}\; 1} = {ɛ_{adh}ɛ_{0}\frac{A_{ring}}{_{adh}}}},} & {{Eqn}.\mspace{14mu} 2}\end{matrix}$

where ∈_(adh) is the relative effective dielectric constant of theadhesive, the foil and the top non-conductive layer of the skin, ∈₀ isthe vacuum permittivity, A_(ring) is the area of the ring capacitor andl_(adh) is the distance between the electrically conductive films andthe conductive part of the skin. When a leak occurs e.g. from the centreof the bandage and outwards, the distance l_(adh), between the topconductive films of the inner ring 7 and the conductive layer of theskin 25 increases, as shown in FIG. 1 b. As a result, C_(adh1)decreases, and according to Eqn.1, f_(res) increases. The presentinvention is therefore capable of detecting parts of the adhesive comingoff the skin and voids or channels created before leakage of body fluidor faeces. Another advantage of the present invention is that enablesdetection of both leakages originating from the peripheral of thebandage and propagating towards the opening 3 and leakages originatingfrom the opening 3 and towards the periphery.

On the other hand, the adhesive also absorbs water. The effectivedielectric constant of the adhesive/foil is approximately ∈_(adh)˜2.5,whilst the dielectric constant of water is as much as ˜80 at DC, andsomewhat smaller at higher frequencies. Therefore, as the adhesiveabsorbs water, according to Eqn.2, C_(adh1) increases, and according toEqn.1, f_(res) decreases. As a consequence of this behaviour, bymonitoring the resonance frequency as a function of time, a leakage canbe discriminated against other changes induced by the environment, suchas changes in temperature, a soaked adhesive due to sweat, mechanicalstresses or other non-leakage induced sensor signals by analysing thetemporal evolution of the resonance frequency.

The dimensions of the bandage illustrated in FIG. 1 may for a wounddressing be in the range of diameters or lengths from 5 mm to 500 mm,more preferably from 30 mm to 150 mm. For an ostomy bandage, thediameters or lengths may be in the range from 30 mm to 300 mm and morepreferably from 50 mm to 150 mm. The periphery of the bandage may havevarious geometric forms such as circular, elliptical, rectangular,polygonal, or other forms. The electrically conductive patterncomprising coils and/or capacitor electrodes are not exclusivelyconfined to spirals for the coils and circles for the capacitorelectrodes as illustrated in the drawings. The coils may form anypattern of conductive leads with mutual electromagnetic coupling and anyarea of a conductive film with a dosed or almost closed loop surroundinga stoma or a wound may form the electrode on a foil or an adhesive.Electrically conductive patterns also include patterns where thecapacitor electrodes and the inductors are spatially distributed forexample by means of a planar coil with a sufficiently broad wire widthforming an electrode which achieves an appropriate capacitance to theconductive layer of the skin of a mammal. Appropriate capacitances meanthat the resonance frequency according to Eqn.1 matches suitablefrequency bands for a reader unit and that it is not dominated byparasitic capacitances. The values of the capacitances and theinductances should thus match the frequency band chosen. For example, atypical value of the inductance is 10 μH, and a typical value of thecombination of C_(adh1) and C_(adh2) is 20 pF yielding a resonancefrequency of f_(res)=11 MHz according to Eqn.1.

According to the present invention, the electrically conductive patternsurrounds the stoma or the wound and it can be designed in various sizesand shapes depending on the size and the shape of the stoma or wound.For a certain resonance frequency within a certain frequency band of thereader circuit, the dimensions of the electrically conductive patternneeds to be adapted to the frequency band. There are a number ofparameters to tune in order to match a certain resonance frequency. Themain parameters are the thickness of the adhesive (l_(adh) in Eqn.2),the area of the ring-capacitance electrode (A_(ring) in Eqn.2) and thenumber of wire turns (n) for the inductance, since in Eqn.1 L_(c)∝m.

The conductivity of skin increases several orders of magnitude when thefrequency changes from DC up to approximately 100 MHz and the differencebetween wet skin and dry skin becomes smaller, the higher the frequency(see e.g. V. Raicu et al. Phys. Med. Biol. 45 (2000) L1-L4). Lowerfrequencies may be preferred for immunity to parasitic effects and theability to penetrate intervening material. Higher frequencies enhancemeasurement accuracy and make the sensor less sensitive to variations inskin humidity.

Standardized and suitable frequency bands for passiveradio-frequency-identification-tags are for example 125 kHz-148 kHz,13.56 MHz, 860-960 MHz, 2.45 GHz where the first two bands are based oninductively coupling, and the latter two bands are usually based on anelectrical dipole coupling. The frequency band in the range from 5-20MHz is most preferable, but other frequency bands can be employed forthe present invention as well.

For a resonant circuit, there are a number of parameters that areaffected when the electrical properties of a dielectric material isaltered by an external condition, which in the present case is a leak.One parameter is the resonance frequency as mentioned above, where aresponse is measured sweeping an applied voltage over a certainfrequency band and performing signal processing of the response asfunction of applied frequency.

Also the harmonic spectrum may be affected. In this case, detection maybe carried out sweeping a receiver through a range of frequencies tocharacterise a harmonic spectrum. A third parameter is thequality-factor (Q-factor) of the circuit (modified by a change inelectrical conductance), where the width of the resonance curve ismeasured or the response to an RF (Radio Frequency) burst and theringing of the resonator is sensed between bursts.

Methods of measuring parameters in resonance circuits are described ine.g. U.S. Pat. No. 6,025,725.

It should be understood that for the two ring-electrodes of the presentinvention, the capacitance between each electrode and the conductivelayer 25 of the skin of a mammal (C_(adh1) and C_(adh2)) should dominateover the capacitance between the two ring-electrodes (C_(w)). As aconsequence, a minimum distance (s) should be provided between the outerradius of the inner electrode (r₂) and the inner radius of the outerelectrode (r₃). The quality factor Q may be limited by the finiteconductance of the skin, and there is an upper limit on the ratiobetween s and the width (w) of any of the two ring-electrodes. The ratios/w is preferable within the interval:

${{1/50} < \frac{s}{w} < 20},$

preferable within

${1/10} < \frac{s}{w} < 5$

and even more preferable within

${1/4} < \frac{s}{w} < 1.$

The quality factor Q may also be limited by the finite conductance ofthe electrodes, which depending on the materials used puts a lower limiton the ratio between the height (h) of the electrodes and w. Inpractical production methods, the height of the electrodes may not beexcessively large. The ratio h/w is preferable:

${10^{- 6} < \frac{h}{w} < 10^{- 1}},$

and more preferable

$10^{- 4} < \frac{h}{w} < {10^{- 2}.}$

Electrode materials may be metals like silver, gold, aluminium or copperor paste of silver or aluminium; conducting polymers like polyaniline,polypyrrole, ethylenedioxythiophene, poly(p-pyridyl vinylene); oramorphous conducting carbon films, films of conducting carbon fibres orpolymer-conducting-carbon-black.

FIGS. 3 a-3 h illustrate schematics of the electrically conductive filmpatterns and cross-sectional views of four different embodiments of thepresent invention with an adhesive 23 attached to the surface of theskin 24 of a mammal and a top-foil 22 on top of the adhesive. FIGS. 3 aand 3 b illustrate a pattern comprising two ring-capacitors 20 and 21connected to a discrete inductor 19 forming a resonance circuit on thetop-foil 22. FIGS. 3 c and 3 d illustrate a pattern comprising tworing-capacitors 26 and 27 and a planar coil 28 connecting thecapacitors. FIGS. 3 e and 3 f illustrate a pattern comprising tworing-capacitors 29 and 30 and a planar coil 31 connecting thecapacitors, but with part of the adhesive comprising a channel 32guiding a leak to the vicinity of the inner ring-capacitor 29. Thechannel 32 may comprise a gap of air or a material with highly liquidguiding properties such as wettable, hydrophilic fibrous materials. Theliquid from an ostomy or from a wound has a high electrical conductivityand it increases the capacitive coupling between the inner conductivering 29 and the conductive part of the skin 25. As a result, a change inthe resonance frequency occurs, and the leak is detected, when itreaches the channel 32. The channel 32 may also act as a “leak stopper”providing a lag in a propagating leak.

The part of the adhesive comprising a channel 32 may alternatively guidea leak to the vicinity of the coil 31 and thereby increase thecapacitive coupling between the coil 31 and the conductive layer of theskin 25 and between the coil and the electrode 29 and/or between thewires of the coil. The function of the leak-guiding channel 32 is toenhance the response sensitivity of the resonance circuit by increasingthe resonance frequency shift, when a leak occurs.

FIGS. 3 g and 3 h illustrate an electrically conductive patterncomprising two ring-capacitors 33 and 34, a planar coil 35 connectingthe capacitors and two additional conducting rings 36 and 37 connectedin parallel to the ring-capacitors 33 and 34 by means of electricalconnectors 38 and 39, respectively, where the two additional conductingrings are in contact with part of the adhesive in which a channel isformed 32′ guiding the leak to be in proximity to the two conductingrings 36 and 37. The liquid from an ostomy or from a wound will form acapacitive or galvanic coupling between the two conducting rings 36 and37. As a result, changes in the resonance frequency and/or the qualityfactor of the circuit occur, and the leak is detected when it reachesthe channel 32′. The channel 32′ may also act as a “leak stopper”providing a delay in a propagating leak. The channel 32′ may comprise agap of air or a material with highly liquid guiding properties such aswettable, hydrophilic fibrous materials. The fabrication of the twoconducting rings 36 and 37 may be made using a folded foil enablingelectrical connection from the top-side of the foil to the bottom sideof the foil. Such methods of folding electrical circuits on foils aredisclosed e.g. in U.S. Pat. No. 6,025,725.

FIGS. 4 a-4 d show partly in perspective and partly cross-sectionalviews of three different embodiments of the present invention withmulti-layer adhesives and electrically conductive patterns. Oneembodiment is illustrated with a perspective view in FIG. 4 a and across-sectional view in FIG. 4 b, the multi-layers comprising a firstadhesive 40 attached to the surface of the skin of a mammal 24, a firstfoil 41, a pattern comprising a set of two ring electrical electrodes 42and 43 forming electrical connections to the wires 95 and 96,respectively, a second foil 41′, a second adhesive or another material45, a third foil 46, an electrically conducting shield layer 44, and atop foil 46′.

As illustrated, the extent of the foils 41 and 41′ may be limited to anarea covering the ring electrodes or they may be extended over the wholebandage. Similarly, the electrically conducting shield layer (44) may beextended over the whole bandage or, it may be bounded by an areasufficiently overlapping the area of the electrodes to preventcapacitive coupling from the environment above the top-foil 46′.

The first and the second foils isolate the electrically conductive filmsfrom the adhesive. When the adhesive 40 absorbs liquid, the foilprevents the liquid from getting into physical contact with andshort-circuiting the electrically conductive films. The two foils 41 and41′ may be any electrically isolating layers. In applications, where theadhesive does not get soaked, they may not be needed. The two ringelectrical electrodes 42 and 43 form capacitors with the conductive skinlayer 25, C_(adh1) and C_(adh2), respectively and a capacitor to theelectrically conducting shield layer 44, C_(w), which approximately canbe expressed as a series connection of two ring-capacitors

$\begin{matrix}{C_{w} = {ɛ_{adhtop}ɛ_{0}\frac{1}{t_{adhtop} + t_{foil}}{\frac{\left( {r_{2}^{2} - r_{1}^{2}} \right)\left( {r_{4}^{2} - r_{3}^{2}} \right)}{r_{2}^{2} - r_{1}^{2} + r_{4}^{2} - r_{3}^{2}}.}}} & {{Eqn}.\mspace{14mu} (3)}\end{matrix}$

In Eqn. 3, ∈_(adhtop) is the effective dielectric constant of the topadhesive and the second and third foil, t_(adhtop) and t_(foil) are thethickness of the second adhesive and the third foil, respectively; r₁ isthe inner radius of the inner conductive ring 43, r₂ is the outer radiusof the inner conductive ring, r₃ is the inner radius of the outerconductive ring, and r₄ is the outer radius of the outer conductivering. The total capacitance seen from the two wires 95 and 96 is givenby C_(w) in parallel with the series connection of C_(adh1) and C_(adh2)(see FIG. 2 b).

Examples of dimensions of a typical design of a leak sensor integratedon an ostomy base plate are as follows. The relative effectivedielectric constant of the adhesives, the foils and the topnon-conductive layer of the skin, ∈_(adh)=∈_(adhtop)=2.5, thickness ofadhesive layer 40, t_(adh)=0.7 mm, thickness of adhesive layer 45,t_(adhtop)=0.3 mm, thickness of third foil, t_(foil)=0.1 mm, the radiiof the conductive rings, r₁=26 mm, r₂=30 mm, r₃=33 mm, r₄=36 mm, thetotal resistance of the skin for the current flowing in the skin betweenthe inner ring-capacitance and the outer ring-capacitance,R_(skin1)+R_(skin2)=68Ω, series resistance of the top conductive layer44, R_(s)=10Ω. Using a total capacitance of

${C_{tot} = {C_{w} + \frac{C_{{adh}\; 1}C_{{adh}\; 2}}{C_{{adh}\; 1} + C_{{adh}\; 2}}}},$

and an inductance of L_(c)=4.7 pH yields according to Eqn.1 a resonancefrequency of f_(res)=13.56 MHz, and a quality factor of Q=fres/Δf=9.7,with Δf being the full-width-half-maximum of the resonance curve. Inpractice, it is necessary to take account of parasitic capacitances andthe leak sensor design needs to be adjusted accordingly. Considering theteachings above such adjustments would be known to a person skilled inthe art.

A leak with an initial air channel of width, height and length ofw_(chan)=8 mm, h_(chan)=1 mm, and l_(chan)=4 mm, respectively, exhibitsa change in C_(adh1) of ΔC_(adh1)=−0.8 pF corresponding to a change inresonance frequency of Δf_(res)=+0.04 MHz. This is a typical measuredvalue of change in resonance for an observed leak. However, earlydetection of a leak often requires measurements of smaller signalchanges. In the art, using a pulse-like excitation signal and amicrocontroller-based circuit, measurements of changes in resonances forresonance-based sensors have been reported to 0.001% achieved inapproximately 40 ms [K. Zeng et al., Rev. Sci. Instrum. Vol. 73 (2002),p. 4375]. For the present example of a leak sensor, an accuracy of thismagnitude corresponds to a measurable change in frequency of 0.0001 MHz,a number, which is a factor of 400 smaller than the leak induced changein resonance frequency as mentioned in the present example.

In practice, the measurement accuracy of the present invention of a leaksensor is not limited by the signal to noise ratio, but rather byinfluence of externally induced signal changes. In order to reduce suchinfluences, an electrically conducting shield layer 44 has beenintroduced in the embodiment as illustrated in FIGS. 4 a-4 b. The shieldlayer eliminates capacitive coupling between the environment above thetop-foil and the electrical conductors 42 and 43. In ostomyapplications, such environmental disturbances could be provided by thehighly conducting liquid in an ostomy bag.

FIGS. 4 c and 4 d show alternative embodiments with compensation foreffects of environmental disturbances in the determination of theresonance frequency. In FIG. 4 c the multi-layers comprise a firstadhesive 47 attached to the outer skin of a mammal 24, a first foil 48,a first electrically conductive pattern comprising two ring-electrodes49 and 50 and a first planar coil 51 electrically connecting theelectrodes, a second foil 48′, a second adhesive or another material 53and a second foil 52 with a second electrically conductive pattern onthe foil comprising a second set of two ring-electrodes 54 and 55 and asecond planar coil 56 electrically connecting the second set ofelectrodes.

In this embodiment of the invention, the first set of electricallyconducting patterns 49, 50 and 51 is employed to detect a leakpropagating at the interface between the surface of the skin 24 and thefirst adhesive 47. The second set of electrically conductive patterns54, 55 and 56 is employed to monitor disturbances from the environmentand subtracting this contribution from the response of the firstresonance circuit 49, 50 and 51. This method enables discriminatingbetween a leak and an environmental disturbance.

There is some mutual inductive and capacitive coupling between the tworesonance circuits, which should be taken into account as known by aperson skilled in the art.

In FIG. 4 d, the multi-layers comprise a first adhesive 67 attached tothe surface of an outer skin of a mammal 24, a first foil 57, a firstelectrically conductive pattern comprising two ring-electrodes 58 and 59and a planar coil 60 electrically connecting the rings, with part of theadhesive comprising a channel 66 guiding a 6 leak to the vicinity of theinner ring-electrode 59, a second foil 57′, a second adhesive or anothermaterial 61 and a top-foil 62 with a second electrically conductivepattern comprising a second set of two ring-electrodes 63 and 64 and asecond planar coil 65 electrically connecting the second set ofelectrodes.

FIG. 5 illustrates another approach of contacting an electricallyconductive pattern embedded in an adhesive as illustrated in FIG. 4 a.FIG. 5 illustrates schematically a piece of foil or the like which maycut along the dashed curves and be folded to make first, second, thirdand fourth sections 220, 221, 222 and 223 overlap each other in fourlayers. The second section 221 has an electrically conductive pattern oftwo ring-electrodes 224 and the third section 222 has an electricallyconducting shield 225 having an outer radius greater than the tworing-electrodes and an inner radius less than the inner radius of any ofthe two ring-electrodes. Electrical contacts to the two ring-electrodeson the third section 222 may be made by means of the conductive strips229 from the second section 221.

For the fabrication of the multi-layer sensor, the sections are foldedas follows, where it should be understood that the printed patternillustrated in FIG. 5 is on the side referred to as the front and theopposite side of the sheet is referred to as the back. In the firststep, the first section 220 is folded along a first folding line 226,between the first and second section, where the front of the firstsection 220 is folded onto the front of the second section 221. In thesecond step, the fourth section 223 is folded along a second foldingline 227, between the third and fourth section, where the front of thefourth section 223 is folded onto the front of the third section 222.Finally, the third and fourth section 222 and 223 are folded along athird folding line 228, between the second and third section, where theback of the third section 222 is folded onto the back of the secondsection 221. An adhesive or another dielectric material preferably withhigh flexibility is optionally disposed on the back of either the secondor third section 221 and 222. The multi-layer structure may then beattached to an adhesive of a dressing or a bandage. In the foldedconfiguration, the cut-out 230 provides means for allowing galvaniccontact to the electrodes of a reader (as will be described later),which may be placed on the cut-out. Alternatively, employing capacitivecoupling to the reader the cut-out 230 may be avoided.

Rather than using one piece of foil, other embodiments also includeconfigurations where the second and third sections 221 and 222 areformed as one layer and the first and fourth sections 220 and 223 areformed as two separate layers. Other embodiments may also includeconfigurations, where the second and third sections 221 and 222 arelaminated together with one layer comprising first and fourth sections220 and 223 and configurations where the first section 220 is a layercoated on the third section 221, and the fourth section 230 is a layercoated on the third section 222. The coating may be performed usingtechniques such as spin-coating, vacuum coating or spray-coating.

FIGS. 6 a and 6 b illustrates a method of fabricating a leak sensor.FIG. 6 a illustrates the bandage produced comprising the threecomponents: a top-foil 68 comprising an electrically conductive pattern69, a middle component of an adhesive 70 and a bottom component of arelease liner 71. In FIG. 6 b the top foil and the adhesive have beenwelded together by heat or another suitable welding method.

This method may also be used to produce configurations such asillustrated in FIG. 4 with multiple layers of foils and adhesives.

The present invention also covers a top-foil comprising atwo-dimensional array of electrically conductive patterns, thetwo-dimensional array being transferred to a series of bandagesseparated by appropriate cutting. The electrically conductive patternsmay be made by, but not limited to, methods such as screen-printing ortampon printing using sliver paste or aluminium paste, orinkjet-produced patterns of solid copper, which may be made by a systemlike the PRECO MetalJet 6000. In addition to the resonance circuit,other electrical components may also be printed on the foil 68 like anRF antenna or a microwave antenna.

The method of the present invention of fabricating a sensor on a foilattached to an adhesive is compatible with existing processes in theproduction of ostomy bandages, base plates and wound dressings.Therefore the method allows for low cost implementation, which is simpleand suitable for mass-production.

The foil comprising the sensor circuit with the electrically conductivepatterns may be produced with an adhesive on the backside and attachedto a release liner. The user removes the release liner and attaches thefoil comprising the sensor circuit to the top of the ostomy bandage orwound dressing. Such a sensor provides flexibility for the user, sinceit can be attached to a number of different products, not limited toparticular ostomy bandages or wound dressings.

FIGS. 7 a and 7 b illustrates two embodiments of the present inventionregarding the coupling to a leak sensor integrated in a bandage of anadhesive 84 of an ostomy bag 83. In FIG. 7 a, the reader unit 85 couplesvia wires 73 and 74 to the electrically conductive pattern comprising aninner ring-electrode 80, and an outer ring-electrode 82. The couplingmay be galvanic, capacitive or inductive. The wires 73 and 74 areattached to the electrically conductive pattern preferably by means of asecond adhesive 75. A coil 81 is preferably included in the reader unit,which connected in series or parallel with the two ring-electrodesconstitutes an electric resonance circuit (see FIG. 3 a). The readerunit comprises a battery or any other kind of energy source 76, amodulation and demodulation circuit transmitting and receivingelectrical signals to and from the electrically conductive patterncombined with a micro-controller with firmware processing the data 77and a data communication unit and drivers for an antenna, LEDs,vibrators or the like 78.

Once the user puts the reader unit onto the bandage 84, the reader makesan initial measurement of the resonance frequency. The reader unit mayhave a button that the user presses, when he/she mounts the reader unit,or more preferably the reader unit starts automatically searching for aresonance frequency, when it has been detached from an ostomy bandagefor example because the user has changed his/her bag. Once the readerunit has found the resonance frequency, it waits a few minutes in orderto let temperature and humidity conditions stabilize. When the resonancefrequency has stabilized, it uses the value of the resonance frequencyas a reference frequency and keeps measuring the resonance frequency.

When the difference between the resonance frequency measured and apreset value exceeds a threshold or exhibits a characteristic temporalpattern associated with a leak, the reader unit 85 sends an alarm,either as an acoustic signal, a vibrating signal, a light signal such asa flashing LED or sends a wireless signal via the antenna 86 to an eventhandling device in a communication network such as a cellular telephone,a personal digital assistant, an iPod, a laptop or a PC. The wirelesssignal may be based on short-range communication standards likeBluetooth, Zigbee or WLAN or any suitable industrial, scientific,medical bands. The reader unit can be reused, when the user changes theostomy bag. In another embodiment, the antenna 86 may be printed ontothe top-foil of the adhesive. The present invention also includesembodiments where the coil 81 is avoided and the reader measures achange in capacitance rather than a change in resonance frequency (seethe following description of FIG. 13).

FIG. 7 b illustrates another embodiment of the present invention, wherethe reader unit 91 comprises an inductive coupling to the coil 90 of theresonance circuit. The reader unit may include a frequency sweepingcircuit and measures the resonance frequency from the frequencydependent coupled impedance. Determination of resonance frequency iswell known in the art; see e.g. O. Akar et al. Sensors and Actuators A95(2001) 29-38). US patent application 2003/0169032 describes some methodsand circuit configurations in determining the resonance frequency basedon the frequency domain. K Zeng at al. Rev. Sci. Inst. Vol. 73 (2002),p. 4375, describe a method and a circuit configuration based on the timedomain. Methods of wireless detection are also known in the art and theyare described e.g. In K. G. Ong et al. Sens. Act. Vol. A93 (2001), p.33.

When the difference between the resonance frequency measured and apreset value exceeds a threshold or exhibits a characteristic temporalpattern, the reader unit sends an alarm, either as an acoustic signal, avibration, a light signal such as a flashing LED or sends a wirelesssignal to a handling device in a communication network such as acellular telephone a personal digital assistant, an iPod, a laptop, a PCor the like.

Similarly to the reader unit 85 in FIG. 7 a, the reader unit 91 in FIG.7 b comprises a battery or any other kind of energy source, a modulationand demodulation circuit combined with a micro-controller with firmwareprocessing the data and a data communication unit and drivers forantenna. LEDs, vibrators or the like.

FIGS. 8 a and 8 b illustrates two methods of transmitting informationabout leak detection from the leak sensor to a service centre or anurse. In FIG. 8 a, the method is based on the sensor configuration inFIG. 7 a. A leak detected by the reader 109 transmits the signal to anevent-handling device 103 using short-range wireless communication. Thehandling device 103 may activate an alarm alerting the user by means ofan acoustic signal, a vibration or the like. The event-handling device103 transfers the signal to a service centre, where a nurse or otherhealth caring personnel is requested to change a bandage or a dressingon a person. The event-handling device may be a cellular telephone, apersonal digital assistant, a laptop, a PC or the like. The signaltransfer to a service centre is carried out by GSM, GPRS, EGSM or DCS orit may be transferred through an intranet or the Internet.

In FIG. 8 b, the method is based on the sensor configuration in FIG. 7b. The wireless detection of a leak by the reader 113 is transmitted toa service centre 116, where a nurse or other health caring personnel isrequested to help the person to change a bandage or a dressing. Thereader unit comprises an event-handling device 114 and transfers thesignal directly to a service centre 116 or via a network to the servicecentre. The communication may be short-range through Bluetooth, Zigbeeor WLAN or it may be long range such as GSM, GPRS, EGSM or DCS. Thehandling device 115 may activate an alarm alerting the user by means ofan acoustic signal, a vibration or the like.

Example 1

In example 1 the sensor response of a leak sensor for an adhesive of anostomy bag with the adhesive attached to the skin of a person isdetermined when a leak is occurs. The leak sensor comprises anelectrically conductive pattern of two ring-electrodes on a top foil ofa bandage and welded to an ostomy bag filled with 0.9% NaCl aqueoussolution simulating body liquid. The configuration is illustrated inFIG. 3 a. The electrically conductive pattern is connected to anexternal coil of 4.7 μH and a sine-wave function generator sweeps thefrequency from 4 to 14 MHz. FIG. 9 shows a plot of an electricalresponse (voltage across the external coil) as function of frequency andFIG. 10 shows a plot of the resonance frequency as function of time,when a leak occurs between the adhesive and the skin of a person at thepoint of time indicated by an arrow 120. From resonance curves acquiredwith a sample frequency of 0.1 Hz, the resonance frequency has beendetermined using a first-momentum fitting algorithm (see eg. C.Thirstrup and W. Zong, Sens. Act. B: Chemical, Vol. 106 (2005), pp.796-802). The RMS noise of the data is 0.007 MHz corresponding toaccuracy in the determination of the resonance frequency of 0.08%.

Example 2

In example 2, a sensor response of a leak sensor for an ostomy bag withan adhesive attached to the skin of a person is determined when theadhesive absorbs moisture. The leak sensor comprises an electricallyconductive pattern of two ring-capacitors on a top-foil of a bandage andwelded to an ostomy bag. The configuration is illustrated in FIG. 3 a.In FIG. 11, the resonance frequency is plotted as function of time whenthe adhesive absorbs sweat from the skin of a cycling person. At thepoint of time 121, the person starts cycling from rest with 74rotations-per-minute and at the point of time 122, the person stopscycling and rests. The resonance frequency has been determined using themethod described in Example 1. Note that the sensor response to anadhesive absorbing sweat from a person is different from the responsefrom a leak (see Example 1). The present invention can thereforediscriminate between leaks and soaked adhesives.

Example 3

In example 3, the sensor response is detected of a leak sensor based ontwo rings connected in parallel to the coil of an electrical resonancecircuit with a top foil of a bandage welded to an ostomy bag filled with0.9% NaCl aqueous solution simulating body liquid. The sensor iselectrically connected to a sine-wave function generator sweeping thefrequency from 1.4-4.6 MHz. The sensor configuration is illustrated inFIG. 3 d apart from the fact that an external coil replaces the planarcoil and that the two ring wires 36 and 37 in this example are touchingthe surface of the skin 24. In FIG. 12, the curve 123 is the responsebefore occurrence of the leak and the curve 124 is after occurrence ofthe leak.

Example 4

Further testing has confirmed the utility of the method and apparatusand shows a high reliability. FIG. 14 a shows a curve plotted using datafrom a set-up comprising a configuration as shown in FIG. 3( a). Thecurve is a plot of the resonance frequency (MHz) as a function of time(min). A person wearing the set-up starts exercising after 8 minutes(t1), provoking a leakage, and after 17 minutes (t2) leakage isobserved. As can be seen from the plot a considerable change in theresonance frequency occurs at t2.

Example 5

In another set-up using a sensor almost similar to that shown in FIG. 3(f), except that an external coil has replaced the integrated planar coilconnected to the inner ring capacitance, data obtained have been plottedin FIG. 14 b. The plot also shows the resonance frequency (MHz) as afunction of time (min). In this set-up the start of provoking a leakage(t1) is begun after 16 minutes where a person wearing the bandage beginsto exercise. A leakage is observed (t2) after 25 minutes. Theconsiderable change in resonance frequency occurring between t1 and t2(in particular between 19 and 20 minutes) is due to the filling of fluidin the channel 32 in FIG. 3( f). Although, at this time leakagetravelling all the way through the bandage has not yet occurred. It isthus possible to obtain a significant indication that a leakage soon iscreated and thereby alert the user or caring nurse in due time.

FIG. 13 a illustrates an embodiment of electrical circuit blocks in theshape of a reader configuration comprising an astable multivibrator(AMV), which is an electrical component known by a person skilled in theart. FIGS. 13 b and 13 c show schematically the variation in thevoltages across the capacitance of the sensor C_(s) (V_(c)) and theoutput of the AMV (V_(osc)) as function of time. The readerconfiguration is based on measuring capacitance change from a change inthe time constant of a resistor-capacitor (R C_(s)) circuit. Thecapacitor C_(s) 237 of the sensor of the present invention and anexternal resistor R 238 are connected to the AMV 236. The capacitor mayhave a design as illustrated schematically in FIG. 3 a or FIG. 4 a, butin this embodiment the inductor is not needed. The reader may beattached to a leak sensor of an ostomy bag as depicted in FIG. 7 a. Asillustrated schematically in FIG. 13 b, the sensor capacitor isperiodically charging up and charging down with a voltage V_(c) changingbetween two threshold values V_(c1) and V_(c2), respectively. When V_(c)reaches V_(c1), the output of the AMV (V_(osc)) goes high, and whenV_(c) reaches V_(c2), Y_(osc) goes low, which is illustratedschematically in FIG. 13 c. The AMV exhibits a square output signalV_(osc) oscillating with a frequency (f), which is inverselyproportional to the product of the capacitor and the resistor, i.e.

$\begin{matrix}{{f \cong {A\; \frac{1}{{RC}_{s}}}},} & {{Eqn}.\mspace{14mu} (4)}\end{matrix}$

with A being a constant.

When C_(s) of the sensor changes, the time of charging up and chargingdown the capacitor changes, and the frequency of the output of the AMVchanges according to Eqn.(4). For an increase in C_(s), it isillustrated schematically in FIG. 13 b that V_(c) changes from the solidplot 231 to the dashed plot 232 and in FIG. 13 c that V_(osc) changesfrom the solid plot 233 to the dashed plot 234. The output from the AMVis transferred to a counter 239, and by counting the number ofoscillations within a certain count time 235, the frequency of V_(osc)may be determined. Monitoring the change in V_(osc), a change in thecapacitance of the sensor may therefore be determined and an occurrenceof a leak may be reported. FIG. 13 c illustrates a situation where thenumber of pulses counted within the count time 235 changes from N to N′.For small variations in sensor capacitance, the corresponding change inthe measured frequency is,

${{\Delta \; f} \cong {{- A}\; \frac{N^{\prime} - N}{N}\frac{1}{{RC}_{s}}}},$

and the corresponding relative change in capacitance is approximatelygiven by

$\begin{matrix}{\frac{\Delta \; C_{s}}{C_{s}} \cong {A\; {\frac{N^{\prime} - N}{N}.}}} & {{Eqn}.\mspace{14mu} (5)}\end{matrix}$

As an alternative to an astable multivibrator, other types ofoscillators may be used, such as a Colpitts oscillator or a Hartleyoscillator, which are also known by the person skilled in the art. Forthe Colpitts oscillator, which is the preferable of the two, theoscillation frequency is determined approximately as in FIG. 1 by aninductor (L_(c)) and the series connection of two capacitors C_(adh1)and C_(adh2).

The output of the counter 239 may be transferred to a micro-controller240, which compares the count with a predetermined time-variation in thecount corresponding to time-variation in the changes of C_(s) and anoccurrence of a leak. The counter may be reset to count at regularintervals. The reset may be carried out internally in the counter or asillustrated in FIG. 13 a, the micro-controller 240 may send a resetsignal 241 to reset the counter at regular intervals.

Alternatively, the counter may run asynchronously with the count beingcaptured by the micro-controller at predefined intervals. The counter orparts of the counter may be integrated in the micro-controller.

The output of the micro-controller 240 is further transferred to acommunication unit 242. The communication unit may report wirelessly thestate of the sensor to an event-handling device, a gate-way or a servicecentre. Alternatively, the communication unit may report an occurrenceof a leak to the user by e.g. an acoustic signal, a vibration signal orthe like.

Examples of component values for a typical design are a sensorcapacitance of C_(s)=20 pF, a resistor R=5 kΩ, a constant A=½, a countrate N=1000 corresponding to a count time of N×RC_(s)/A=200 μs, whichcan be achieved for a counter with a bit resolution 10 bit, and a changein the count of N′−N=1. According to Eqn.(5), with these numbers, theresolution of capacitance change is

$\frac{\Delta \; C_{s}}{C_{s}} = {0.0005.}$

The resolution of measuring the capacitance can be increased e.g. byincreasing the bit resolution of the counter. The count time and thefrequency need to be adjusted accordingly. As an example, with a 16 bitresolution the count rate is N=65536 corresponding to a count time of 13ms and a

$\frac{\Delta \; C_{s}}{C_{s}}\mspace{14mu} {of}\mspace{14mu} 8 \times {10^{- 8}.}$

The electrical coupling 243 and 243′ between the capacitance of thesensor and the AMV may be capacitive or galvanic.

There are a number of other methods of measuring capacitance from thetime constant of a resistor-capacitor (RC_(s)) circuit. Such methodsinclude: an ac bridge method, charge and decharge methods, a switchedcapacitor technique and capacitance-to-phase angle conversion (see e.g.Ref. A. Ashrafl et al., Rev. Sci. Instrum., Vol. 70 (1999), p. 3483 andreferences therein).

In cases where ΔC_(s)<<C_(s), compensation of the fixed offsetcapacitance can be made and higher measurement accuracy can be achieved.One measurement scheme employs a direct-to-digital capacitive sensorreadout circuit based on a capacitance-controlled relaxation oscillatorin which the fixed offset is cancelled within an analogue oscillator.The reference capacitance (C_(ref)) is subtracted from the sensorcapacitance (C_(s)) multiplied by a constant (b) and measurements of apulse duration proportional to R′×(C_(s)−bC_(ref)) with R′ being aresistor are carried out. The change in pulse duration changes the dutycycle of a square-wave oscillator, which is converted to a digitalnumber by a counter (see Z. Ignjatovic and M. F. Bodco, IEEE SensorsJournal, Vol. 5 (2005), p. 403 and other measurement schemes reported inreferences therein).

FIGS. 15 a-15 d illustrate several embodiments of attachment of anostomy bag 153; 158; 165 and 175 (pouch) onto a bandage 136; 137; 138and 139 (adhesive wafer) comprising different embodiments of adhesiveleak sensors. The pouches are welded to the topfoils 161; 166 and 176 ofthe bandages.

The embodiment illustrated in FIG. 15( a) comprises an adhesive 160attached to the surface of the outer skin of a person 24, a topfoil 161,a first electrically conductive pattern comprising two ring-electrodes162 and 163, a pouch 153 attached to the topfoil 161 by a weld seam 154.From the opening 152, a substance 157 may fill or partly fill the pouch.

The substance 157 is typically a stool from a stoma, which iselectrically conductive, and causes a considerable increase in thecapacitive coupling between the two ring-electrodes 162 and 163.

FIG. 15( b) illustrates an embodiment with a pouch attached to thetopfoil of an adhesive. Isolation means 159 are provided around thepouch 158. The isolation means may comprise a material with a lowdielectric value such as polyethylene, fluorinated benzoate copolymer ora substance embedding dry air such as a nanoporous dielectric material,a foam, a non-woven or the like. Additionally, the outer side of thepouch 158 or the part of the pouch covering the electrodes 162 and 163may be coated by an electrically conductive film 134. Thus, a fixedcapacitance is introduced comprising the first electrically conductivepattern, i.e. the inner and outer ring-electrodes 163 and 162, thedielectric material 159 and the electrically conductive coating of theouter side of the pouch 134. The coating may be evaporated, sputtered orsprayed aluminium, conducting polymers like polyaniline, polypyrrole,ethylenedioxythiophene, poly(p-pyridyl vinylene); or amorphousconducting carbon films, films of conducting carbon fibres orpolymer-conducting-carbon-black. Alternatively, the foil of the pouch158 may be laminated with a conductive layer 134. In addition the foilof the pouch 158 and the conductive layer 134 may be laminated with theisolation means 159.

The isolation means reduces the capacitive coupling through theelectrically conductive substance 157. A first weld seam 155 attachesthe pouch to the topfoil 161 of the bandage 137. The first weld seam isprovided between the opening 152 and the inner ring electrode 163. Asecond weld seem 156 attaching another area of the pouch to the topfoil161 is provided between the outer circumference 135 of the bandage andthe outer ring electrode 162. Thus a pocket 170 is formed containing theinner and outer electrode ring. This advantageously prevents capacitivecoupling, for example when the substance 157 is displaced over theelectrically conductive pattern 162 and 163 or a region in the vicinitythereof.

FIG. 15( c) illustrates an alternative embodiment with an adhesive 167attached to the surface of the outer skin 24 of a person, a first foil168, a first electrically conductive pattern comprising an outerring-electrode 172 and an inner ring-electrode 173, a second adhesive orother dielectric material 171, a topfoil 166, a shield layer 164comprising a second electrically conductive pattern, a pouch 165attached to the topfoil 166 of the bandage 138 by a weld seam 169. Atthe weld position, a groove 184 in the shield layer 164 may be made toenable the weld. The second electrically conductive pattern formed inthe shield layer 164 prevents capacitive coupling between theelectrically conductive substance 157 and the first electricallyconductive pattern. Instead a fixed capacitance is introduced consistingof the first electrically conductive pattern, i.e. the inner and outerring-electrodes 173 and 172, the dielectric material 171 and the shieldlayer 164 comprising the second electrically conductive pattern. For afirst electrically conductive pattern of two ring-electrodes, the fixedcapacitance is approximately given by eqn. (3).

The second electrically conductive pattern can be formed of manydifferent types of materials, for example metals like silver, gold,aluminium or copper or paste of silver or aluminium; conducting polymerslike polyaniline, polypyrrole, ethylenedioxythiophene, poly(p-pyridylvinylene); or amorphous conducting carbon films, films of conductingcarbon fibres or polymer-conducting-carbon-black. Materials may also bedoped semiconductors such as tin oxide (SnO₂), zinc oxide (ZnO₂), indiumtin oxide (ITO) or the like. The second electrically conductive patternmay be made by, but not limited to, methods such as screen-printing ortampon printing using silver paste or aluminium paste, orinkjet-produced patterns of solid copper or ink of highly conductingsubstances.

FIG. 15( d) illustrates an alternative embodiment, where the firstelectrically conductive pattern formed of outer ring-electrode 182 andinner ring-electrode 183, is positioned close to the opening 152.

The bandage is attached to the surface 24 of the outer layer of the skinby a first adhesive 177 which is disposed on a first foil 178 arrangedopposite the skin, on the distal side of the first adhesive. The outerring-electrode 182 and the inner ring-electrode 183 forming the firstelectrically conductive pattern are printed on top of the first foil.Between the first foil 178 and the top film 176 there is disposed asecond adhesive 181 or other dielectric material. On the top film 176,opposite the second adhesive there is provided a shield layer 174 in theform of a second electrically conductive pattern.

The extent of the foils 161, 168, and 178 may be limited to an areacovering the first electrically conductive pattern or they may beextended over the whole bandage or beyond the extent of the bandage.Similarly, the second electrically conductive patterns 164 and 174 maybe extended over the whole bandage or they may be bounded by an areasufficiently overlapping the area of the first electrically conductivepattern to prevent capacitive coupling from an electrically conductivesubstance 157.

By providing a groove 185 in the shield layer 174 a weld seam 179 can beprovided therein attaching the pouch 175 to the top film 176 of thebandage 139. The weld seam 179 is positioned between the twoelectrically conductive rings 182 and 183 when looking at the bandage ina top view. This reduces the effect of capacitive coupling at the weldseam.

The weld seams 154, 155, 156, 169 and 179 may be made by means of heatwelding, ultrasonic welding, high frequency welding, transmission laserwelding or other suitable welding techniques. Alternatively, the weldseams may be achieved by means of gluing.

FIGS. 16 a and 16 b illustrate an example of the effect of the shieldlayers described with respect to FIGS. 15 a-15 d. FIGS. 16 a and 16 bplot the changes in the adhesive leak sensor signal (i.e. the resonancefrequency [MHz]) as a result of lifting up in an ostomy bag welded tothe adhesive and filled with a liquid (80 ml 0.9% NaCl aqueous solution)for a period of time [min].

FIG. 16 a is a plot for a sensor configuration without shield layer asin FIG. 15 a. Initially, the liquid causes capacitive coupling to theadhesive leak sensor. When the bag is lifted up at t1, the capacitivecoupling is removed, and when the bag is lifted down again at t2, thecapacitive coupling is re-established. This shows a significant changein the resonance frequency between t1 and t2.

FIG. 16 b is a similar plot for a sensor configuration with shieldlayer, as shown in FIG. 15 c, where the bag is lifted up at t1 andlifted back down again at t2.

For an ostomy product with a configuration as illustrated schematicallyin FIG. 15 a, when the bag is lifted down, the liquid causes capacitivecoupling between the two electrically conductive rings 162 and 163.Lifting the bag up in the direction as indicated by the arrow 180,removes this capacitive coupling.

For the configuration as illustrated in FIG. 15 a without shield layer,this creates large changes in the sensor signal (˜0.5 MHz) as observedin FIG. 16 a between t1 and t2. Depending on the sensor assembly andset-up, these changes may be similar to or even larger than a leak forthis configuration. For a configuration with a shield layer as shown inFIG. 15 c, there is no or negligible effect of lifting the bag up anddown in the direction as indicated by arrow 180 in FIG. 15 c and asobserved in FIG. 16 b between t1 and t2.

FIG. 17 a-17 d shows an alternative embodiment of a bandage of thepresent invention. FIG. 17 a shows in top view the conductive patternsfor the two ring capacitors 142 and 143. FIG. 17 b shows in top view theelectrical shield layer 144. FIG. 17 c illustrates the alignment of theconductive patterns for the two ring electrodes 142 and 143, the shieldlayer 144 and a reader 147. FIG. 17 d shows in cross section theembodiment along line A-A in FIG. 17 c.

Electrical connection to the outer ring electrode 142 is made by the twoconductive fingers 148 and 148′ and electrical connection to the innerring electrode 143 is made by the conductive finger 149. The reader 147is electrically connected to the two ring capacitors 142 and 143 via theconductive fingers 148, 148′ and 149. The connection may be galvanic orcapacitive. Alternatively, the fingers may comprise a coil-like patternand the electrical connection may be inductive.

The foil 141 comprising the electrically conductive pattern whereon theouter and inner ring electrodes 142,143 are printed extends beyond theperiphery of the shield layer. It is embedded between a first adhesivelayer 140 and a second dielectric layer 145, which may be a secondadhesive. An additional layer 151 may support the foil mechanically. Theadditional layer 151 may be an extension of the first adhesive 140 andit may or maybe not be attached to the surface of the skin 24. Thetopfoil 146 comprises the shield layer 144.

The distance 150 between the two fingers 148′ and 149, and theequivalent distance between 148 and 149, should be large enough toexhibit small parasitic capacitance and inductance, but small enough toenable safe detection of a potential leak at this position. The distanceis preferably between 1/20 and 5 times the distance between the outerradius of the inner ring electrode and the inner radius of the outerring electrode, more preferably between ⅛ and 2 times this distance andeven more preferably between ¼ and 1 times this distance.

What is claimed is:
 1. A dressing adapted to be applied to a surface ofan at least partly electrically conductive object, the dressingcomprising a first adhesive for attaching the dressing to the at leastpartly electrically conductive object, and at least two electrodesadapted to be arranged in a distance from the at least partlyelectrically conductive object so that a first capacitor is adapted tobe formed between a first electrode and the at least partly electricallyconductive object, and a second capacitor is adapted to be formedbetween a second electrode and the at least partly electricallyconductive object.
 2. The dressing according to claim 1, furthercomprising an electrically conducting shield for reducing ambientdisturbances of the first and second capacitors.
 3. The dressingaccording to claim 2, wherein the electrically conducting shield forms apocket containing the first and second electrodes.
 4. The dressingaccording to claim 2, wherein the electrically conducting shieldcomprises an electrically conductive film.
 5. The dressing according toclaim 4, wherein the electrically conductive film comprises materialsselected from the group consisting of: aluminium, conductive polymers,amorphous conducting carbon, conducting carbon fibers and polymerconducting carbon black.
 6. The dressing according to claim 2, whereinthe electrically conducting shield is provided on an outer side of anassociated pouch adapted to hold a substance.
 7. The dressing accordingto claim 6, wherein a dielectric material is provided between the firstand second electrodes, and the electrically conducting shield.
 8. Thedressing according to claim 7, wherein at least part of the dielectricmaterial is provided on the electrically conducting shield of theassociated pouch adapted to hold a substance.
 9. The dressing accordingto claim 2, wherein the electrically conducting shield is laminated withan associated pouch adapted to hold a substance.
 10. The dressingaccording to claim 9, wherein isolation means is laminated with theelectrically conducting shield of the associated pouch adapted to hold asubstance.
 11. The dressing according to claim 10, wherein at least partof the laminated isolation means is provided between the first andsecond electrodes, and the electrically conducting shield.
 12. Thedressing according to claim 2, further comprising a second adhesive atleast partly encapsulating the first and second electrodes so as to forma distance between the first and second electrodes and the electricallyconducting shield.
 13. The dressing according to claim 12, furthercomprising a foil at least partly secured to the second adhesive. 14.The dressing according to claim 2, further comprising a dielectricmaterial at least partly encapsulating the first and second electrodesso as to form a distance between the first and second electrodes and theelectrically conducting shield.
 15. The dressing according to claim 14,further comprising a foil at least partly secured to the dielectricmaterial.
 16. The dressing according to claim 13, wherein theelectrically conducting shield is at least partly secured to the foil.17. The dressing according to claim 16, wherein the electricallyconducting shield comprises materials selected from the group consistingof: metals, conductive polymers, amorphous conducting carbon, conductingcarbon fibers, polymer conducting carbon black and doped semiconductors.18. An ostomy collection bag attached to the dressing according toclaim
 1. 19. An ostomy collection bag comprising a dressing and a pouchattached thereto, the dressing being adapted to be applied to a surfaceof an at least partly electrically conductive object, the dressingcomprising a first adhesive for attaching the dressing to the at leastpartly electrically conductive object, at least two electrodes adaptedto be arranged in a distance from the at least partly electricallyconductive object so that a first capacitor is adapted to be formedbetween a first electrode and the at least partly electricallyconductive object, and a second capacitor is adapted to be formedbetween a second electrode and the at least partly electricallyconductive object, means for applying a voltage to the first and secondelectrodes, means for detecting changes in a capacitance between atleast one of the first and the second electrode and the at least partlyelectrically conductive object, and an alarm to be activated when thechanges of the capacitance reach a predetermined value, wherein anelectrically conducting shield for reducing ambient disturbances of thefirst and second capacitors is laminated with the pouch.
 20. The ostomycollection bag according to claim 19, wherein isolation means islaminated with the electrically conducting shield of the pouch.
 21. Theostomy collection bag according to claim 20, wherein at least part ofthe laminated isolation means is provided between the first and secondelectrodes, and the electrically conducting shield.
 22. The ostomycollection bag according to claim 21, wherein the electricallyconductive film comprises materials selected from the group consistingof: aluminium, conductive polymers, amorphous conducting carbon,conducting carbon fibers and polymer conducting carbon black.
 23. Asensor assembly comprising a reader for detecting detachment of thedressing according to claim 1, wherein the reader comprises means ofreceiving electrical signals from the dressing.
 24. The sensor assemblyaccording to claim 23, wherein the reader further comprises means oftransmitting electrical modulation signals to the dressing.
 25. Thesensor assembly according to claim 23, wherein the reader iscapacitively coupled to the first and second electrode.
 26. The sensorassembly according to claim 23, wherein the reader is galvanicallycoupled to the first and second electrode.
 27. The sensor assemblyaccording to claim 23, wherein the reader is inductively coupled to aninductor connected in series or parallel with the first and secondelectrode.
 28. The sensor assembly according to claim 23, wherein thereader further comprises means for transmitting information representingthe electrical signals from the dressing to a data processing unit.